By the time women are 34 weeks pregnant, their unborn babies can respond to the sound of their mother’s voice reciting a familiar nursery rhyme, the researchers report.

“The mother’s voice is the predominant source of sensory stimulation in the developing fetus,” Charlene Krueger, nursing researcher and associate professor in the University of Florida’s College of Nursing, said in a university news release. “This research highlights just how sophisticated the third trimester fetus really is and suggests that a mother’s voice is involved in the development of early learning and memory capabilities. This could potentially affect how we approach the care and stimulation of the preterm infant.”

In conducting the study, published recently in the journal Infant Behavior and Development, the researchers asked 32 pregnant women who spoke English as their first language to recite a rhyme to their babies out loud a few times a day for six weeks.

The women recited a nursery rhyme or passage as directed from 28 to 34 weeks of pregnancy. At 34 weeks, the women were asked to stop reciting their rhyme. During weeks 28, 32, 33 and 34 of the pregnancy, researchers tested the babies for evidence of learning.

Using a fetal heart monitor, the researchers analyzed the babies’ heart rate. A slowing of the heart rate, they explained, would be a sign of learning.

During the test, some unborn babies were played a recording of the same rhyme that had been recited by their mother. This time however, the rhyme was spoken by an unfamiliar female voice.

To determine if the babies were responding to their mother’s voice or a familiar pattern of speech, a second group of unborn babies heard a different rhyme recorded by a female stranger.

The study revealed the babies’ heart rate began to respond to the familiar rhyme by 34 weeks of pregnancy. By this time, they had heard their mother recite the rhyme regularly for six weeks. The researchers identified a slight slowing of the babies’ heart rate for up to four weeks after their mothers stopping reciting their rhyme.

By 38 weeks, the group of babies that heard a stranger recite the rhyme spoken by their mother at home had a more significant response to the familiar words. The researchers found there was a deeper and more sustained slowing of their heart rate. Meanwhile, the babies that heard a new rhyme experienced a quickening of their heart rate.

“This study helped us understand more about how early a fetus could learn a passage of speech and whether the passage could be remembered weeks later even without daily exposure to it,” Krueger said. “This could have implications to those preterm infants who are born before 37 weeks of age and the impact an intervention such as their mother’s voice may have on influencing better outcomes in this high-risk population.”